Strain crystallizable elastomers are very desirable as tire polymers for improved tire properties, such as improved tire traction characteristics, improved flex characteristics and improved tear resistance. Unfortunately, polyisoprene which is prepared utilizing conventional procedures is not strain crystallizable due to its amorphous structure. Such 3,4-polyisoprenes can be easily prepared using an organolithium catalyst in conjunction with a polar modifier. For instance, Natta, Porri, and Carbonaro, 77 Makromolecular Chemistry 126 (1964) discloses a catalyst for preparing atactic 3,4-polyisoprene.
A technique for preparing crystallizable 3,4-polyisoprene is disclosed by Sun and Wang, 2 Acta Polymerica Sinica 145 (1988). These Chinese researchers disclose that crystalline 3,4-polyisoprene containing about 70% 3,4-microstructure and about 30% cis-1,4-microstructure can be prepared using a catalyst system containing iron acetyl acetonate, a trialkyl aluminum compound, and an amine modifier in benzene. However, the utilization of this catalyst system developed by the Chinese results in the formation of gel and poor yields.
U.S. Pat. No. 3,725,373 to Yoo discloses a catalyst system which contains (a) an iron source, (b) an electron donor ligand such as 2,2-dipyridyl or 1,10-phenanthroline, and (c) a reducing agent. Yoo indicates that the iron compound can be in the form of a hydrate or alcohol complex. The catalyst system disclosed by Yoo is reported to be useful in homopolymerizing acrylonitrile.